1. Field of the Invention
This invention relates to electroless metal plating and more particularly to a process for the chemical reduction deposition of metal coatings on surfaces involving a new and improved catalytic activation of the surface or surfaces prior to the chemical reduction metal deposition. Additionally this invention relates to new and improved catalyst solutions which are substantially free of colloidal metal particles, and to a method for preparing the catalyst solutions.
2. Description of the Prior Art
Prior activation systems for electroless metal deposition consisted of separate solutions of a stannous salt, e.g. stannous chloride, and a noble metal salt, e.g. palladous chloride. The stannous salt solution and noble metal salt solution were true solutions, and the activation was a two step process with separate application of the stannous salt solution followed by application of the palladous salt solution with water rinsing between steps. Colloidal tin and colloidal silver-containing baths have also been used as activation systems for electroless metal plating. Combination colloidal catalyst systems of noble metal salts and tin salts have also been used heretofor for activation substrates for electroless metal plating. The colloidal activation or catalyst systems referred to above, although giving good results in certain respects, leave room for improvement from the standpoint of stability of the colloidal catalyst-containing baths and effectiveness in catalyzing the surface or surfaces intended to be electrolessly metal plated. Thus the colloidal activator or catalyst systems have colloidal catalyst particles of such large size that contacting and catalyzing of certain surface or surfaces intended to be electrolessly metal plated tends to be impossible, for example in penetrating, contacting and activating the surfaces of narrow, deep recesses such as those often found in multi-layer printed circuit boards and the surfaces of small holes, especially small blind holes, and crevices encountered in plating other plastics.
U.S. Pat. No. 3,011,920 discloses the use of a liquid containing colloidal particles of the catalytic metal dispersed therein for activating a surface for electroless deposition of metal thereon. The colloidal catalyst sol is obtained by admixing a noble metal salt, a tin salt, and a hydro-halide acid. Although the colloidal catalyst sol gives satisfactory results for activation, the colloidal catalyst sol leaves appreciable room for improvement in activating the surfaces of printed circuit boards, due to the colloidal catalyst particles being of such large size as to prevent the particles from penetrating and hence contacting and activating the surfaces of the narrow, deep recesses. The colloidal catalyst sol of U.S. Pat. No. 3,011,920 also leaves room for improvement from the standpoint of inherent stability of the catalyst or activator composition.
The colloidal catalyst sols and baths of the prior art also leave room for improvement with regard to tolerance to "dragged in" CrO.sub.3 - and H.sub.2 SO.sub.4 -containing aqueous conditioner solution. The plastisol-coated plated racks employed in the plating cycle tend to have cracks, fissures and pores in the plastisol, especially after the racks are older and have been in use for an appreciable time, and the conditioning solution may be retained in the cracks, fissures and pores and consequently "dragged in" to the activator bath despite water rinsing after the conditioning and prior to immersion of the racked articles to be plated into the activator bath. The presence of the "dragged in" CrO.sub.3 - and H.sub.2 SO.sub.4 -containing conditioner solution in the prior colloidal catalyst sols and baths may result in a premature or quite early decomposition of the colloidal catalyst or activator sol to the extent that it will no longer function to effectively catalyze the article surface or surfaces to be electrolessly metal plated.
Moreover, use of the colloidal catalyst sol of U.S. Pat. No. 3,011,920 in the plating cycle requires a special post-activation treatment of the catalyzed article surface or surfaces with a post-activation solution which may be hazardous as hereafter disclosed, to remove protective colloid and/or deflocculating agent from the deposited colloidal catalytic metal particles. The post-activation solution employed for this special post-activation treatment may contain perchloric acid, and such post-activator solution could present an explosion and fire hazard due to the perchloric acid undergoing a spontaneous and explosive decomposition under certain conditions.
Catalyst solutions containing a soluble Lewis base-modified noble metal-tin halide complex and free or substantially free of colloidal catalyst metal are disclosed and claimed in U.S. Pat. No. 3,767,583, assigned to the common assignee. Although such catalyst solutions of U.S. Pat. No. 3,767,583 constitute an improvement in certain respects over the colloidal catalyst sol of the prior art for catalyzing surfaces destined to be electrolessly metal plated and are eminently suited for use in plating on plastics generally, the catalyst solutions of U.S. Pat. No. 3,767,583 leave room for improvement in catalyzing the walls of the through holes of through hole printed circuit boards and of multi-layer printed circuit boards. A portion or portions of the surface area of the through hole walls were usually not activated using the catalyst solution of U.S. Pat. No. 3,767,583, and consequently these non-activated surface portions of the through walls were not metal plated and hence not rendered electrically conductive during the subsequent electroless metal plating step.